The present invention relates generally to communications systems, and more specifically to a noise generation system.
Efforts in the design of integrated circuits for radio frequency (RF) communication systems generally focus on improving performance, reducing cost or a combination thereof. Some applications require the generation of random noise, referred to as white noise. White noise is produced indiscriminately across a frequency range, with each frequency within the range having an approximately equal level of noise power, subject to statistical deviations.
Amplifier linearity is often characterized by measuring the intermodulation distortion (IMD) products resulting from the input of two tones to an amplifier. A second test employs inputting a single modulated carrier and measuring the power that leaks into adjacent spectral channels. Comparing the wanted signal power to the leakage in adjacent channels is referred to as the Adjacent Channel Power Ratio, ACPR, test.
There are many applications where many carriers are required or where it is much more cost effective to amplify multiple signals with a single amplifier. The tests for IMD and ACPR may not indicate an amplifier""s performance when many signals are input. It is difficult to generate a large number of input signals because the test apparatus can easily generate intermodulation products, which will then be amplified and taint results measured at the output.
One application of a noise generation assembly lies in component testing. A commonly used test to characterize multi-carrier power amplifiers is called the Noise Power Ratio (NPR) test. The NPR test loads an amplifier with broadband noise except for a xe2x80x9cnotchedxe2x80x9d quiet region in which the level of signal is measured. The broadband noise is a surrogate for a large number of carrier signals. If the amplifier is perfectly linear then the noise will be amplified but there will be no signal xe2x80x9cleakagexe2x80x9d into the notched region. The level of signal present in the notched region when the loading is applied minus the level present with no loading indicates the degree of amplifier distortion. The Noise Power Ratio is the ratio of output power, measured in a bandwidth of interest, between the high noise and notched region. A mechanical filter that is configured manually for each test generally provides the notched region.
Linearity is a critical parameter for most transmitters because there is a significant cost impact both in components and input power to provide highly linear amplification. The consequence of non-linearities is distortion in wanted signals which may lead to received errors and unwanted or out-of-band emissions. These emissions can interfere with signals in a transmitter""s band or with other systems and are strictly regulated by national and international regulations and standards.
A second application of noise generators is the jamming or interdiction of signals within a selected range of frequencies. A jamming signal attempts to overwhelm communications within its associated wideband frequency range by flooding the communications medium, such as cable, wire, or air, with noise. If not used judiciously, the jamming signal will indiscriminately affect both desired and undesired communications.
In accordance with one aspect of the present invention, a selective noise generation system is provided. A delta-sigma modulator receives digital input and produces a digital output. A digital-to-analog converter converts the digital output into an analog output. The analog output comprises at least one low noise or xe2x80x9cquietxe2x80x9d frequency band and at least one frequency band of noise. The low noise frequency bands have respective associated shapes and center frequencies. A frequency control controls the delta-sigma modulator to alter one of the respective center frequency and the shape of the low noise frequency bands. The digital input can be approximately zero in which case it is not an information bearing signal or it can be designed to provide desired, information bearing signals within the xe2x80x9cquietxe2x80x9d band.
In accordance with another aspect of the present invention, a method of selectively generating noise within a frequency range is provided. A digital input, having a first word size, is quantized to produce a digital output signal having a second word size. The first word size is larger than the second word size. The digital input is processed as to distribute noise associated with quantizing the digital signal across the frequency range, such that the quantization noise is spread across at least one frequency band of quasi-random noise and is substantiality reduced in at least one low noise frequency band having associated frequency characteristics. The quantized digital output signal is converted into an analog signal. The frequency characteristics of the at least one frequency band are altered.